The present invention relates to a set-up method for a linearising circuit and particularly but not exclusively to a so-called feed-forward linearising system.
The present invention is concerned with linearised circuits and particularly but not exclusively with linearised power amplifiers, that is amplifiers which do not generate distortion products and thus produce output signals containing only the spectral components of the input signal. Outside the input signal spectrum, signals are attenuated as far as possible. Such amplifiers are used particularly in mobile communications networks, where transmission systems operate within designated frequency bands which are termed herein transmission bands or communication bands. For example, according to one mobile communications standard, transmission channels each having a band width of 5 MHz may lie within a communication band of 2.11 GHz to 2.17 GHz, that is having an overall frequency width of 60 MHz. Thus, the power amplifier which amplifies these transmission channels prior to transmission via an RF antenna is effective if it amplifies signals in this frequency range, but attenuates signals outside that frequency range to avoid, as far as possible, interfering signals being transmitted.
Linear power amplifiers are subject to the generation of interference or distortion products which, if supplied with the output signal, cause unwanted interference. In an attempt to reduce this interference as far as possible, a feed-forward system is used when the amplifier is operating. According to this feed-forward system, a pilot tone is supplied to the amplifier in addition to the input signal which is to be amplified. At the output of the amplifier, a coupler takes a sample of the output signal. A pilot signal detection circuit detects the pilot tone, which has now been subject to the interference products, and supplies this to an adaptive control unit which compares the extracted pilot tone with the input pilot tone to generate a correction signal for the power amplifier. The purpose of the correction signal is to adjust the feed-forward system of the amplifier so that the original distortion or interference products generated within the amplifier are substantially cancelled.
It can be seen that such a linearisation method relies on the pilot signals being easily detectable at all times. If this is not the case, problems arise when trying to extract the modified pilot signal from the output signal. The comparison between the output pilot signal and the input pilot signal is then flawed which results in the feed-forward system of the amplifier being compromised and will not effectively linearise the amplifier. It will be understood that the accuracy and speed of the linearisation are closely associated with the ability to detect the pilot signal and thus the ratio of the amplitudes of the pilot signal and the interference signal is highly important to the optimal operation of the amplifier.
The problem of detection of the pilot signal is greatest when the amplifier is initially switched on. At this time, the spectrum of the output signal is highly broadened since the adaptive control unit takes some time to adjust the feed-forward system to optimise the amplitude and phase controls to linearise the amplifier. Consequently, when the amplifier is initially switched on, the amplified output signal contains high levels of interference which reduces the ratio of pilot signal amplitude to interference signal amplitude to the point where it becomes difficult to accurately detect the pilot signal. Since the pilot signals are naturally limited to lower power levels than those of the amplified signals, in some instances the amplitude of the interference may be greater than the amplitude of the pilot signal, further hindering pilot signal detection.
It is thus desirable to establish an initialisation procedure for a linear amplifier which ensures that the pilot signals are easily detectable at all times, even during the initial start-up of the amplifier.
According to one aspect of the present invention there is provided a method of setting up a linearising circuit in which a pilot tone is used both during set-up and during normal operation for interference cancellation, wherein the set-up method comprises:
supplying an input signal to the linearising circuit at a low, starting power level;
detecting the pilot tone in an output signal of the linearising circuit;
comparing the detected pilot tone with the supplied pilot tone to determine the interference products of the linearising circuit, and using said interference products to generate a correction signal for interference cancellation at the linearising circuit;
increasing the power level of the input signal to the linearising circuit during set-up while continuing to detect the pilot tone for interference cancellation until the power level of the input signal attains an operational level for normal use.
The correction signal can be applied with an adaptation factor which determines the speed of the correction process. The present invention allows for this adaptation factor to be set to a high value during start-up while the power level of the input signal is at the low level. The adaptation factor can then be reduced to its normal operational level for normal use.
Another aspect of the present invention provides a feed forward linearising system comprising:
a linearising circuit arranged to receive an input signal and a pilot tone for interference cancellation purposes;
detection means for detecting the pilot tone in an output signal of the linearising circuit and for comparing it with the supplied pilot tone to determine a correction signal;
adaptation means for applying the correction signal to the linearising circuit for interference cancellation purposes; and
attenuation means for setting the power level of the input signal to a low, starting level for set-up purposes, and for increasing the power level of the input signal during set-up to an operational level for normal use.
In the described embodiment the linearising circuit is a linearised power amplifier.
The invention is particularly useful when utilised within a transmission system for a mobile communications network. Thus, the invention also provides a transmission system for a mobile communications network comprising:
means for generating an information signal representing data to be transmitted via the mobile communications network;
a mixer for combining said information signal with a carrier signal to generate a combined signal for transmission purposes;
a power amplifier for amplifying said combined signal to a power level adequate for transmission via an RF antenna; and
a start-up unit for said linear power amplifier which attenuates the combined signal prior to supplying it to the power amplifier for the purposes of setting up the power amplifier prior to its normal use.
According to the embodiment described herein, an initialisation method for a feed-forward power amplifier is disclosed in which pilot tones are used for amplitude and phase control. The described method comprises the steps of:
setting the power level of the signal to be amplified at a low level when the amplifier is switched on,
setting to a high value an adaptation factor which determines the speed of the adaptation process,
increasing the power level of the signal to be amplified step by step in order to approach the required power level (for normal operation),
reducing the adaptation factor during the process to make the adaptation less susceptible to interference during normal use, and
switching off the start-up procedure to an idle state after the amplifier has settled into stable operation and the required power level of the input signal has been reached.
Although described in the context of a linearised power amplifier, the invention can be used in any lineariser if a pilot tone is used for distortion corrections. The technique described herein allows for effective and quick distortion cancellation in a feed-forward lineariser. This is achieved by starting adjustments of the feed-forward loop and adaptation process at a low input power and gradually increasing the power when proper cancellation is reached.